For many purposes in agriculture and related endeavors it is desired to treat plants with exogenous chemical substances of various kinds. Many exogenous chemical substances are applied to foliage (i.e., leaves and other non-woody above-ground parts) of a plant, and have a site of action in the plant either close to or remote from the locus of application. Such substances are referred to herein as foliar-applied exogenous chemical substances.
Typically, when an exogenous chemical substance is applied to foliage by plant treatment processes known in the art, only a small portion of the amount applied reaches sites of action in the plant where a desired biological activity of the exogenous chemical substance can be usefully expressed. It is therefore a major desideratum in agriculture and related endeavors to enhance the efficiency of delivery of foliar-applied exogenous chemical substances to their sites of action in plants, and thereby to enhance the biological effectiveness of the exogenous chemical substance for the purpose for which the exogenous chemical substance is used.
Application to foliage of an exogenous chemical substance by processes known in the art does not universally result in inefficient delivery to sites of action. In some situations such processes provide excellent biological effectiveness, even at a low use rate of the exogenous chemical substance. In other situations the same processes, using the same rate of the same exogenous chemical substance, provide inadequate biological effectiveness. Thus, these processes are inconsistent in the result they provide, or they cannot be relied upon to provide the desired result.
A problem is that it is seldom possible to identify in advance those situations where good biological effectiveness will be obtained, partly because so many factors influence efficiency of delivery. These factors include weather (temperature, relative humidity, daylength, cloudiness, precipitation, wind, etc.) preceding, during and following application, soil conditions (fertility, aeration, etc.), plant growth stage, health and physiological status, equipment-related inaccuracies in application, and other factors. Therefore, to help ensure reliable or consistent biological effectiveness of a foliar-applied exogenous chemical substance, the user typically applies the substance at a higher rate than is truly necessary in the majority of situations.
Variability in biological effectiveness in field conditions is an especially troublesome problem in the case of exogenous chemical substances that are acids, and are typically formulated as water-soluble salts in which the exogenous chemical substance is present in an anionic form. Sometimes by converting such acid substances to esters, this variability can be moderated; however, in many cases esters show reduced biological effectiveness, for example due to inadequate conversion back to the parent acid once inside the treated plant. There remains a strong need for enhanced biological effectiveness, and enhanced reliability of biological effectiveness, of foliar-applied exogenous chemical substances, particularly anionic exogenous chemical substances.
The term "anionic exogenous chemical substance" as used herein means an exogenous chemical substance whose molecular structure includes one or more acid, or proton-donating, sites, and is therefore capable of forming an anion in the presence of a proton acceptor. The term therefore embraces substances that are zwitterionic. In describing an exogenous chemical substance as "anionic" herein, it is not implied that the exogenous chemical substance is necessarily in anionic form or that it is dissociated.
Benefits of a process providing greater reliability of biological effectiveness include an ability to reduce rates of application of exogenous chemical substances without sacrificing consistency of biological effectiveness. Pressures felt by the agricultural industry to reduce pesticide, particularly herbicide, usage are well evidenced by symposia on the subject, such as that held in 1993 by the Weed Science Society of America and documented in Weed Technology 8, 331-386 (1994). Reduced use rates bring rewards not only environmentally but also economically, as the cost per unit area treated decreases.
Foliar-applied exogenous chemical substances have frequently been applied together with amphiphilic materials, particularly amphiphilic surface-active agents, otherwise known as surfactants. Surfactants can influence biological effectiveness of a foliar-applied exogenous chemical substance in numerous ways.
When a dilute aqueous composition of an exogenous chemical substance is applied to foliage by conventional hydraulic spraying, the presence of surfactant in the dilute aqueous composition can alter the size distribution of the spray droplets, typically increasing the percentage of spray volume in the form of small droplets and reducing the percentage of spray volume in the form of large droplets. As smaller droplets have lower momentum than larger droplets, these smaller droplets are less likely to rebound from a foliar surface and consequently are more likely to be retained on that surface. Spray retention can also be facilitated by adhesion of surfactant molecules in a spray droplet to the foliar surface, which in most plants is waxy and hydrophobic. This adhesion reduces not only rebound but also run-off of spray droplets from the foliar surface. Surfactants also tend to increase the area of contact between a spray droplet and a foliar surface, and in many cases enhance penetration of an exogenous chemical substance from the droplet into and through cuticles of leaves to reach internal leaf tissues.
Through these and perhaps other effects, amphiphilic materials including surfactants have long been known to increase the biological effectiveness of exogenous chemical substances. It is therefore commonplace for one or more surfactants to be included in commercial formulations of foliar-applied exogenous chemical substances, even in formulations that do not require the presence of surfactants for acceptable physical stability or handling properties, for example as emulsifying or suspending agents or dispersants.
One of the most extensively studied of foliar-applied anionic exogenous chemical substances, from the point of view of the role of surfactants in enhancing biological effectiveness, is the herbicide glyphosate. As well as being a phytotoxic agent, glyphosate has been used as a plant growth regulator.
Glyphosate(N-phosphonomethylglycine) in its strict sense is an acid compound, but the word "glyphosate" is herein used in a less restrictive sense, except where the context dictates otherwise, to encompass not only glyphosate acid but also salts, adducts and esters thereof, and compounds which are converted to glyphosate in plant tissues or which otherwise provide glyphosate ions. In most commercial formulations of glyphosate, the glyphosate is present as a water-soluble salt. In this respect, glyphosate is typical of most exogenous chemical substances that are acids or that form anions.
Herbicidal salts of glyphosate are disclosed, for example, in U.S. Pat. No. 3,799,758 to Franz, U.S. Pat. No. 3,853,530 to Franz, U.S. Pat. No. 4,140,513 to Prill, U.S. Pat. No. 4,315,765 to Large, U.S. Pat. No. 4,405,531 to Franz, U.S. Pat. No. 4,481,026 to Prisbylla and U.S. Pat. No. 4,507,250 to Bakel. Typical of such salts are alkali metal, for example sodium and potassium, salts; ammonium salt; and numerous salts having an ammonium, sulfonium or sulfoxonium cation substituted with 1-3 organic groups containing in total 1-6 carbon atoms, for example dimethylammonium, isopropylammonium, ethanolammonium and trimethylsulfonium salts.
Commercial formulations of glyphosate salts include, for example, Roundup.RTM., Accord.RTM., Roundup.RTM. Ultra and Roundup.RTM. Xtra herbicides of Monsanto Company, which contain the isopropylammonium salt, Roundup.RTM. Dry and Rival.RTM. herbicides of Monsanto Company, which contain the ammonium salt, Roundup.RTM. Geoforce herbicide of Monsanto Company, which contains the sodium salt, and Touchdown.RTM. herbicide of Zeneca, which contains the trimethylsulfonium salt.
Glyphosate as a herbicide has many advantages, particularly environmental advantages including biodegradability and low ecotoxicity. However, studies have shown that even the most biologically effective formulations of glyphosate presently in use do not deliver glyphosate efficiently to sites in the plant where the glyphosate exerts its phytotoxic effect. Typically, only a small fraction of the applied herbicide arrives at such sites.
The small fraction of applied glyphosate which reaches sites of phytotoxic action is related to the fact that the glyphosate must go through several barriers. Among these, one of the most important is believed to be the lipophilic cuticle on the foliar surface to which the glyphosate is applied. It has therefore been theorized that it would be desirable to place the glyphosate into an amphiphilic medium which would provide greater compatibility between the lipophilic cuticle and the hydrophilic glyphosate, and thereby facilitate penetration of glyphosate into and through the cuticle. Similar thinking has been applied to other exogenous chemical substances, particularly those typically formulated as water-soluble salts.
That the concept of an amphiphilic medium as an aid to cuticular penetration and thereby enhanced biological effectiveness, for example of glyphosate, has validity is demonstrated by many studies in which foliar uptake or effectiveness has been enhanced by surfactants. An extensive study by Wyrill & Burnside, Weed Science 25, 275-287, 1977 led to a conclusion that "an effective surfactant is a critical component of any glyphosate spray mixture", but noted great variation among surfactant types in the degree of enhancement of herbicidal effectiveness afforded. The authors cautioned that "effectiveness of surfactant combinations was quite variable and difficult to predict" and that "the indiscriminate addition of surfactants or wetting agents to glyphosate spray mixtures which already contain a surfactant should be avoided". The Wyrill & Burnside study generally involved application of surfactants at very high rates relative to the rates of glyphosate applied. Under these conditions, and on the particular plant species they studied, the authors found that in general, cationic surfactants gave greater enhancement of the herbicidal effectiveness of glyphosate than did nonionic surfactants. It is well accepted by those of skill in the art that relative effectiveness of surfactants in enhancing biological activity of an exogenous chemical substance, particularly glyphosate, at high surfactant levels does not permit reliable prediction of relative effectiveness at much lower surfactant levels, as in the realm of the present invention.
Data are reported in International Publication No. WO 98/06259 for a wide variety of cationic, nonionic, anionic and amphoteric surfactants applied either in mixture with, or in sequence following, a glyphosate composition.
The term "alkyl", as used herein in relation to hydrophobic moieties of surfactants, has the broad meaning, as conventionally applied in surfactant art, of aliphatic hydrocarbyl, embracing unsaturated groups, for example alkenyl and alkynyl groups, as well as saturated alkyl groups, unless the context demands otherwise.
Surfactants having a hydrophilic moiety comprising one or more protonatable amino groups or cationic ammonium groups together with a total of 1 to about 100 oxyethylene units in one or more oxyethylene chains constitute a favored selection of surfactants useful in formulating glyphosate and other anionic exogenous chemical substances. For example, commercial glyphosate herbicide products marketed under the trademark Roundup.RTM. have been formulated with surfactant compositions based on polyoxyethylene C.sub.8-22 alkylamines. For example, the surfactant composition MON 0818 of Monsanto Company, which has been extensively used in the formulation of Roundup.RTM. herbicide, contains a polyoxyethylene tallowamine having an average of about 15 oxyethylene units per molecule.
Numerous patents disclose compositions comprising glyphosate and an oxyethylene or polyoxyethylene amine or ammonium surfactant.
U.S. Pat. No. 5,668,085 to Forbes et al. discloses compositions comprising glyphosate and a polyoxyethylene C.sub.8-22 alkylamine surfactant having an average of up to about 12 oxyethylene units per molecule. Australian Patent Application No. 57565/90 discloses compositions comprising glyphosate and a polyoxyethylene C.sub.8-22 alkyldiaminopropane surfactant. U.S. Pat. No. 5,317,003 to Kassebaum & Berk discloses compositions comprising glyphosate and a quaternary polyoxyethylene C.sub.6-14 alkylmethylammonium surfactant having about 5 to about 50 oxyethylene units per molecule. U.S. Pat. No. 5,652,197 to Claude et al. discloses compositions comprising glyphosate and a quaternary polyoxypropylene oxyethylene tri-(C.sub.1-3 alkyl)ammonium surfactant having 2 to 20 oxypropylene units per molecule. U.S. Pat. No. 5,118,444 to Nguyen discloses compositions comprising glyphosate and a polyoxyethylene C.sub.6-20 alkylamine oxide surfactant having about 5 to about 25 oxyethylene units per molecule. U.S. Pat. No. 5,750,468 to Wright discloses compositions comprising glyphosate and a polyoxyethylene tertiary alkyletheramine, polyoxyethylene quaternary alkyletherammonium or polyoxyethylene alkyletheramine oxide surfactant. French Patent Application No. 2 648 316 discloses compositions comprising glyphosate and a polyoxyethylene N-alkyl-1,3-diaminopropane surfactant.
Polyoxyethylene C.sub.16-22 alkylether surfactants have been less frequently disclosed in compositions with glyphosate, and generally at surfactant to glyphosate weight ratios outside the realm of the present invention. For example, European Patent No. 0 206 537 discloses solid compositions comprising glyphosate and Plurafac.TM. A-39 surfactant of BASF, which is a polyoxyethylene C.sub.16-18 alkylether surfactant having an average of about 55 oxyethylene units per molecule. The lowest weight ratio of Plurafac.TM. A-39 to glyphosate acid equivalent disclosed therein can be calculated as about 1.16:1 (composition 12 of Table IV of the cited patent).
Wyrill & Burnside, op. cit., disclose plant treatment compositions comprising glyphosate and a 1:1 mixture of a polyoxyethylene alkylether surfactant identified as Plurafac.TM. A-46 with polyoxyethylene alkylamine surfactants Ethomeen.TM. T/15 of Akzo and MON 0818 of Monsanto, at surfactant to glyphosate ratios far outside the realm of the present invention. Probably because the surfactant concentration in the compositions was so high (1% weight/volume), no significant benefit was evident for the mixtures over the alkylamine surfactants alone.
At lower surfactant to glyphosate weight ratios, strong enhancement of glyphosate herbicidal effectiveness has been reported for mixtures of a polyoxyethylene alkylamine or alkylammonium surfactant and a polyoxyethylene alkylether surfactant, where the alkylether is derived from a secondary alcohol, as in International Publication No. WO 95/16351, a Guerbet alcohol, as in U.S. Pat. No. 5,663,117 to Warner, or an acetylenic diol, as in U.S. Pat. No. 5,639,711 to Kassebaum et al. European Patent Application No. 0 582 561 discloses a solid granular glyphosate composition containing a polyoxyethylene quaternary alkylammonium surfactant (Ethoquad.TM. 18/25 of Akzo) and a polyoxyethylene C.sub.13 alkylether surfactant (Trycol.TM. 5943 of Henkel) but does not report herbicidal effectiveness of this composition.
Townson, in her Ph.D. thesis, Influence of formulation and application variables in relation to the performance of glyphosate and imazapyrfor control of Imperata cylindrica (L.) Raeuschel, University of Bristol, U.K., 312 pp., 1990, compared polyoxyethylene C.sub.16-18 alkylethers having respectively 3, 12 and 19 oxyethylene units for enhancement of foliar retention, uptake, translocation and herbicidal effectiveness of glyphosate and imazapyr. Comparison was also made with polyoxyethylene alkylethers having shorter alkyl chain lengths (C.sub.9-11, C,.sub.12-15 and C.sub.13-15). The study further included polyoxyethylene alkylamine surfactants but no blends of alkylether and amine surfactants were tested.
It is an objective of the present invention to provide a new formulation of an exogenous chemical substance, in particular an anionic exogenous chemical substance, that can provide superior biological effectiveness when applied to foliage of a plant.
Another object of the invention is to provide a useful alternative to existing formulations of anionic exogenous chemical substances.